The term wind shear refers to a meteorological phenomenon in which an isolated moving mass of air has a significant vertical component of motion. Although frequently associated with microbursts or localized rain squalls, winds circulating downwardly in a column may occur without any apparent warning in otherwise relatively calm, clear air. An aircraft flying through a wind shear condition is not subject to any significant danger, so long as its altitude is sufficient; however, if the aircraft is either taking off or on a landing approach, the pilot's response to the unexpected rate-of-change of air movement may cause the aircraft to lose altitude. In extreme wind shear conditions, at low altitude, the actions of the pilot may even result in the aircraft crashing into the ground.
FIG. 1 graphically illustrates how lateral air movement resulting from the downward movement of an air mass spilling away from the ground creates a dangerous wind shear condition with respect to an aircraft on a landing approach. As the aircraft flies into the downdraft, it is first exposed to a horizontal component of the air mass moving toward the aircraft. Accordingly, the pilot notices an apparent increase in airspeed and increased lift at point A. To remain on the approach glide path, a pilot's instinctive reaction would be to throttle back the engines and push forward on the yoke, bringing the aircraft nose down. However, at point B, the aircraft encounters a horizontal component of the air mass moving in the same direction as the aircraft. This shear condition causes a loss of airspeed and altitude which can have disastrous results unless the pilot reacts quickly to increase the throttle setting and regain a safe altitude, returning onto the glidepath at point C.
Wind shear detectors have been developed to avoid the above scenario by alerting the crew of an aircraft of an incipient wind shear condition. If the pilot is thus warned of the wind shear condition, which causes a loss of total airplane energy, he can avoid his instinctive reaction to reduce throttle in order to maintain airspeed and pitch the airplane down, and instead begin a proper management of the available airplane energy as he is trained to do. Typical prior art wind shear detection and warning systems are disclosed in U.S. Pat. Nos. 3,618,002; 4,593,285; and 4,728,951; and in PCT WO 87/06043.
Each aircraft manufacturer has established a reference airspeed, V.sub.REF, for their aircraft during an approach for landing, which varies as a function of load and configuration criteria. The reference airspeed is conservatively selected, and is well above the stall airspeed of the aircraft. Furthermore, the recommended landing speed is at or a few knots above V.sub.REF. The wind shear alerting systems are designed to warn of an incipient wind shear condition based on the aircraft landing at the recommended approach (or takeoff) airspeed with respect to V.sub.REF. However, on a landing approach, pilots often increase the speed of their aircraft by ten to twenty knots above the reference airspeed to reduce the effect of any wind shear that is present, since the aircraft is easier to control in a wind shear condition at a higher airspeed. At the higher than recommended landing approach airspeeds, pilots have noted and complained of numerous nuisance wind shear alerts, where the effect of wind shear on the aircraft was too small (at the higher initial airspeed of the aircraft) to justify the pilot being alerted.
Once the wind shear alert is sounded, the pilot is instructed to abort the landing and go around. None of the prior art wind shear detection and warning systems provide any mechanism to compensate for the aircraft's airspeed being above the recommended level. Since any warning system that produces significant false alarms may eventually be ignored--with potentially disastrous results, the problem of unnecessary wind shear alerts is more important than may at first appear evident.